Blood vessels, the esophagus, intestines, endocrine gland ducts, the urethra and other lumens are all subject to strictures i.e., a narrowing or occlusion of the lumen. Strictures can be caused by a variety of traumatic or organic disorders and symptoms can range from mild irritation and discomfort to paralysis and death.
Most life threatening stenoses are associated with the cardiovascular system and are often treated by performing a percutaneous coronary intervention (PCI) such as balloon angioplasty (also referred to as percutaneous transluminal coronary angioplasty or PTCA). Balloon angioplasty is performed by threading a slender balloon-tipped catheter from an artery in the groin to a trouble spot in an artery of the heart. Once in position, the balloon is inflated to thereby dilate (widen) the narrowed coronary artery so that blood can flow more easily. Unfortunately, experience has shown that three to six months after PCI, many patients experience restenosis (some estimates place the number at between a third and half of all patients experience restenosis). Injury to the arterial wall during PCI is believed to be the initiating event that causes restenosis. The resulting stricture is often formed from vascular smooth muscle cell proliferation and extracellular matrix secretion at the injured site. Restenosis is also a major problem in non-coronary artery disease including the carotid, femoral, iliac, popliteal and renal arteries.
Other non-vascular tubular structures can also suffer from stenosis due to a variety of causes such as inflammation, neoplasm, and benign intimal hyperplasia. Some strictures such as those in the esophagus or intestines, may be surgically removed and the lumen repaired by anastomosis. The smaller transluminal spaces associated with ducts and vessels may also be repaired in this fashion. Unfortunately, restenosis caused by intimal hyperplasia is common in these situations.
Aging men often suffer from stenosis of the urethra that results in diminished urine flow rates. The most frequent cause is enlargement of the prostate gland (e.g., benign prostatic hypertrophy or BPH). In this disease, the internal lobes of the prostate slowly enlarge and progressively occlude the urethral lumen. A number of therapeutic options are available for treating an enlarged prostrate. These include watchful waiting (no treatment), several drugs, a variety of so-called “less invasive” therapies, and transurethral resection of the prostate (TURP)—long considered the gold standard.
In the urethra, a circumferential band of fibrous scar tissue may progressively contract and narrow the lumen thereby reducing the urine flow rate. A stricture of this type may be congenital or may result from urethral trauma or disease. These strictures were traditionally treated by dilation with sounds or bougies. More recently, balloon catheters have been used to mechanically dilate the lumen. Surgical urethrotomy is currently the preferred treatment, but restenosis remains a significant problem.
Stents were developed, at least in part, to attempt to minimize the occurrence of restenosis. Stents can generally be thought of as a form of mechanical scaffolding that holds the occluded lumen open. There are two general types of stents: permanent and temporary. Temporary stents can be further subdivided into removable and absorbable.
Permanent stents are used where long term structural support or restenosis prevention is required, or in cases where surgical removal of the implanted stent is impractical. Permanent stents are usually made from metals such as Phynox, 316 stainless steel, MP35N alloy, and superelastic Nitinol (nickel-titanium).
Although stents are used primarily in the vasculature, stents may also be used to hold any bodily lumen open. For example, stents may be used as temporary devices to prevent closure of a recently opened urethra following minimally invasive procedures to treat an occlusion due to an enlarged prostate. These procedures often result in a post treatment edema and urethral obstruction. In these cases, the stent is typically not covered with tissue (epithelialized) prior to removal.
Temporary absorbable stents can be made from a wide range of synthetic biocompatible polymers depending on the physical qualities desired. Representative biocompatible polymers include polyanhydrides, polycaprolactone, polyglycolides, polylactides, and polyphosphate esters.
Recently, a number of biocompatible, bioresorbable materials have been used in stent development and in situ drug delivery development. These stents are designed and made from copolymers which unfortunately may not provide the desired physical properties required to hold the lumen open for a sufficient time period for healing to occur. Accordingly, it would be desirable to provide an improved biocompatible, bioresorbable stent that has the desired physical properties necessary to hold the lumen open for a time sufficient to promote healing. Also, it would be desirable to provide a stent that has a substrate that facilitates the growth or regeneration of tissue as the substrate is removed by or incorporated into the patient's body.